Investigations On The Conservative Factors For Burnup Credit

The concept of taking credit for reduction in reactivity due to fuel burnup is commonly referred to as burnup credit. The successful application of burnup credit can reduce cost in spent fuel storage and transport as well as enhance productivity in reprocessing. However, the conservative calculation results are affected by many factors. The key factors are the depletion history effects, conservative factors in criticality calculation, sensitivity and uncertainty analysis of nuclear cross section data, the influence by analytic tool, etc. In this study, the OECD/NEA burnup credit criticality benchmarks problems are investigated by use of modules of in SCALE5.1 licensed software package. Conclusions on the conservativeness of these influencing factors are made and the explanations in basic physics are given.First of all, each fuel assembly experiences unique depletion process, which gives rise to diversity of nuclides concentration and reactivity in each fuel assembly. Dismissing the specific characters in different fuel assemblies, burnup credit considers a set of bounding depletion parameters to guarantee the conservativeness of the results. The depletion history effects mainly include depletion parameters, specific power density, existence of burnable absorbers and control rods effects. Secondly, when we apply the burnup credit technology to perform criticality safety analysis for spent fuel storage and transportation problems, it is important to confirm that all the adopted conditions are adequate to cover the severest conditions. This paper studies important factors in criticality calculation, including nuclides selection, cooling time and end effect. Moreover, the mechanism of end effect are also presented.Thirdly, whether in depletion or criticality calculation, it is inevitable to utilize cross section data in calculation. Howercer, cross section data generated by experiments and theories exists uncertainties. Therefore, the basic methodology for sensitivity analysis in SCALE5.1 and some sensitivity and uncertainty analysis for infinite cell would be performed in this study.Finally, effects caused by different nuclear data libraries and correlative modules are also discussed in the thesis.This work is supported by the National 863 projects of China via research project 2009AA050706.